Priority is claimed with respect to German Application No. 11 538.9-14 filed in Germany on Mar. 16, 1999, the disclosure of which is incorporated herein by reference.
The invention relates to an apparatus for changing the transmission ratio of a continuously adjustable transmission, e.g. a CVT, as part of a cruise control system for motor vehicles.
Cruise controllers combined with step-change gearboxes are known and are widely used, especially in larger motor vehicles. Fundamentally, a cruise controller controls the speed of travel as a function of a stipulated desired value by influencing the fuel supply. While this presents no problems during straight-ahead travel and hill climbing, there is the risk, particularly during downhill travel, that the speed of travel will continue to increase due to the downhill slope, despite the fuel supply being reduced to zero. Generally, therefore overrun cut-off is first of all activated and, if this is still not sufficient, the step-change gearbox is changed down by one gear or even by several gears in order to increase the engine braking effect.
Cruise controllers have hitherto not been used with continuously variable transmissions and the possibilities for transferring the known type of cruise control to continuously variable transmissions are limited due to the lack of gear stages.
DE 43 30 391 A1 and EP 0 658 711 A1 have disclosed continuously variable transmissions, but these were not combined with a cruise controller. All that is provided there are measures which ensure that when the accelerator pedal is in the idle-speed position during downhill travel, the speed of travel at the time when downhill travel began is held or held at least as tar as possible by changing the transmission ratio.
One object of the present invention is to design a cruise controller in a vehicle with a continuously adjustable transmission in such a way that comfortable retention or reduction of the speed of travel is made possible even on a downhill slope and when the desired vehicle speed is reduced by the driver.
By means of the apparatus according to the invention as part of a cruise control system combined with a continuously adjustable transmission, it is possible to adjust the transmission ratio steadily and continuously and in such a way that it is matched exactly to the load conditions without impairment by gear-change jerks. It is particularly advantageous here that, when the overrun phase ends after downhill travel, exactly the same transmission ratio as was previously effective is re-established. There is thus no discernible gear-change operation and this offers the ideal conditions for maximum comfort.
The stipulated value, which is compared with the actual value of the speed of travel to form the difference value, is preferably the value of the speed of travel at the beginning of overrun operation and/or the decreasing desired value for the speed of travel in the case of a specifically intended vehicle deceleration by means of an operating element of the cruise controller.
For controlling the transmission ratio, use is preferably made of a P controller which has no time delay behaviour and hence avoids overshooting or undershooting.
The controller for the formation of a change value for the transmission ratio has an engine-speed limiting device which advantageously prevents a further change in the transmission ratio when a stipulatable maximum speed of the drive motor for the motor vehicle is reached in order to ensure that a troublesome noise level is not reached. This controller furthermore advantageously has arranged on its output side a characteristic-curve modification stage which, in the case of small change rates for the transmission ratio, transmits reduced versions of these values and/or, in the case of larger change values, transmits increased versions of these values. This means that, in the case of a small vehicle-speed overshoot, the change in the transmission ratio can be made moderate but, as the downhill slope increases, can be made progressive, thus allowing optimum adaptation to the conditions of the downhill slope to achieve maximum comfort.
The stipulated value for this controller is furthermore expediently provided with an offset in order to allow better adaptation of the effective controller intervention to the specifics of the vehicle.
A limiting device for the transmission ratio furthermore contributes to an increase in comfort since this measure too helps to avoid the unwanted imposition of troublesome noise.
To increase safety and avoid an unwanted and impermissible transmission intervention when the conditions envisaged for this have not been met, e.g. due to a fault or a faulty signal, there is a cut-off device which prevents a desired value being stipulated for the transmission ratio by the cruise controller, this cut-off device only permitting this desired-value stipulation when the actual value of the speed of travel exceeds the stipulated value.
To limit transmission interventions to what is necessary, it is expedient if, in overrun operation, an overrun cut-off is activated first before the transmission ratio is changed by the cruise controller. This can be preferably switched on with a hysteresis at a stipulatable difference between the desired value and the actual value of the speed of travel and exerts a certain braking action. The change in the transmission ratio is then triggered at a larger stipulatable difference between the desired value and the actuable value of the speed of travel.